Engraved diagram: method of causing a ball to revolve on a pivot over a heated cauldron of water. A bent tube connects the hollow ball and the cauldron, which is covered by a lid. Opposite the end of the tube is placed a pivot, resting on the lid. The ball contains two bent pipes, communicating with it at the opposite extremities of a diameter, and bent in opposite directions, the bends being at right angles. As the cauldron gets hot it will be found that the steam, entering the ball, pases out through the bent tubes towards the lid, and causes the ball to revolve, as in the case of the dancing figures. Illustration from 'Gli artifitiosi et curiosi moti spiritali di Herrone' (commonly called 'Pneumatics') by Hero of Alexandria, published in Ferrara in 1589.
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